Download The Origin of the Solar System and Other Planetary Systems

The Origin of the
Solar System
and Other
Planetary
Systems
Modeling Planet Formation
Boundary Conditions
Nebular Hypothesis
Fixing Problems
Role of Catastrophes
Planets of Other Stars
Modeling Planet Formation
Any model must explain:
1. Planets are relatively isolated in space
2. Planetary orbits are nearly circular
3. Planetary orbits all lie in (nearly) the same plane
4. Direction of orbital motion is the same as direction of Sun’s
rotation (prograde revolution)
5. Direction of the rotation most planets is the same as the Sun’s
rotation (prograde rotation)
6. Most satellite orbits are also in the same sense (prograde
revolution)
7. Solar system is highly differentiated
8. Asteroids are very old, and not like either inner or outer planets
9. Kuiper belt, asteroid-sized icy bodies beyond the orbit of
Neptune
10. Oort cloud is similar to Kuiper belt in composition, but farther out
and with random orbits
Nebular Hypothesis
1. The solar system began as a rotating
cloud of gas and dust.
2. It began to collapse.
3. Conservation of linear and angular
momentum caused the collapsing
cloud to flatten into a disk.
4. This process increased the density of
the medium.
5. The center remained semi spherical
because of a concentration of heat.
6. As temperatures decreased, gases
could condense into liquids and
solids.
7. Planetesimals formed from the
accretion of condensed material and
collided to form planets.
The Condensation of Solids
 To compare densities of
planets, one must compensate
for compression from the
planet’s gravity
 Only condensed materials
could stick together to form
planets
 The temperature in the
protostellar cloud decreased
outward.
 A cooler protostellar cloud ⇒
matter with lower vaporization
points condense ⇒ change of
chemical composition
progressing outward in the
solar system
Temperature in the Solar Nebula
The temperature in the nebula determines where various materials
condensed into small grains of solids or drops of liquids
Formation and
Growth of
Planetesimals
 After condensation, planet formation
continues with the sticking together
of colliding drops and grains
(accretion) to form planetesimals
 Planetesimals grow from both
condensation and accretion.
 Planetesimals (few cm to km in size)
collide to form planets.
 Gravitational instabilities may have
helped in the growth of
planetesimals into protoplanets.
Terrestrial Planets
Accretion theory:
 A large interstellar cloud or
nebula of gas and dust
starts to contract, heating
up from the loss of
gravitational potential
energy
 The Sun forms in center;
dust provides condensation
nuclei, around which
planetesimals form
 As planets grow, they
sweep up planetesimals
near them, called accretion
Differentiation in the Solar System
Terrestrial (rocky) planets formed near the Sun,
where temperatures were higher – ices could not
condense there
The Growth of
Protoplanets
The simplest form of planet growth:
Unchanged composition of accreted
matter over time
As rocks melted, heavier elements
sank to the center ⇒ differentiation
Gas is released from a hot interior
(outgassing) ⇒ production of a
secondary atmosphere
A variation of this scenario:
Gradual change of accreted grain
composition resulting from the
cooling nebula which allows volatile
materials to condense after the
accretion of refractory materials—
differential accretion
The Giant Planet Problem
Two problems for the theory of giant planet formation:
1) Observations of extrasolar planets indicate that giant
planets are common.
2) The radiation from nearby massive stars causes
protoplanetary disks to evaporate quickly (typically
within ~100,000 years).
⇒ Too short for giant planets to grow!
Solution:
Computer simulations show that giant planets can
grow by direct gas accretion without forming rocky
planetesimals.
Giant Planets
Giant planets:
 Started accreting
like terrestrial
planets from more
abundant ices
 They grew quickly
 Once they were
large enough, they
probably captured
gas directly from
the contracting
nebula
 They may have
formed from
instabilities in the
outer, cool regions
of the nebula
Solar Rotation Problem
Conservation of angular momentum requires that the
product of radius and rotation rate must be constant for
a particle. This implies that the Sun should be rotating
much more rapidly than it does.
Two bodies of
the same
mass and
different radii
can have the
same angular
momentum, if
the body of
smaller radius
has a
comparably
larger rotation
speed
Solar Rotation Problem
As it collapsed, the solar nebula had to conserve
its angular momentum
However, now, the Sun has almost none of the
solar system’s angular momentum
 Jupiter alone accounts for 60%
 Four giant planets account for more than 99%
Why doesn’t the Sun rotate more rapidly?
Solar Rotation Problem
Hypothesis: The Sun transferred most of its angular
momentum to outer planets (and interstellar space) as a
result of friction from the magnetic drag of field lines
passing through a conducting plasma (magnetic braking).
Solar Rotation Problem
The plausibility of magnetic
breaking from a strong solar
wind can be found in T Tauri
stars which are in a highly
active phase of their
evolution and have strong
stellar winds. Besides being
a source of magnetic
braking, these winds also
sweep away the gas disk,
leaving the planetesimals
and gas giants.
Asteroid Problem
Why is there an asteroid belt?
 Orbits mostly between Mars and Jupiter
 Jupiter’s gravity inhibited planetesimals from
condensing into a planet or accreting onto an
existing one although a few dwarf planets were
formed
 This implies that most asteroids are
essentially planetesimals left over from the
initial formation of the solar system
 Some asteroid families are debris of larger
asteroids that were broken apart in a collision
Kuiper Belt and Oort Cloud
Icy planetesimals far from the Sun have remained as Kuiper
belt bodies. Some of these were ejected into distant orbits by
gravitational interaction with the giant planets forming the
Oort cloud. Oort cloud objects have very eccentric orbits and
occasionally appear in the inner solar system as comets.
Clearing the Nebula
Remains of the protosolar nebula were cleared away by:
• Radiation pressure of the Sun
• Sweeping-up of space debris by planets
• Solar wind
• Ejection by close encounters with planets
Surfaces of the Moon and Mercury show evidence for heavy
bombardment by asteroids sweeping-up some the debris.
Timeline of Solar System Formation
The Role of Catastrophes
Condensation-accretion theory explains the 10 properties of the
solar system mentioned at the beginning.
However, there are special cases not explained by the theory.
1. Mercury’s large metallic core may be the result of a collision
between two planetesimals, where much of the mantle was lost.
2. Two large bodies may have merged to form Venus.
3. Earth–Moon system may have formed after a collision.
4. A late collision may have caused Mars’ north–south asymmetry and
stripped most of its atmosphere.
5. Uranus’ tilted axis may be the result of a glancing collision.
6. Miranda may have been almost destroyed in a collision.
7. Interactions between giant protoplanets and planetesimals could be
responsible for irregular satellites.
8. Pluto is probably a large Kuiper-belt object.
The Role of Catastrophes
 Many of these explanations have one thing in
common – a catastrophic, or near-catastrophic,
collision at a critical time during formation.
 Normally, one does not like to explain things by
calling on one-time events, but it is clear that
the early solar system involved almost constant
collisions. Some must have been exceptional.
 Collisions can give predictable results when
large numbers of them can be treated
statistically. However, there are always
statistical outliers that produce extraordinary
results.
Planets of Other Stars
The theory of planet formation has planets evolving from a stellar nebula.
Since most stars form
from a stellar nebula
⇒ Many stars should
have planets!
Most other planets
cannot be imaged
directly.
How are they found?
We look for
“wobbling” motion of
the star around the
common center of
mass with its
planetary system.
Searching for Extrasolar Planets
Planets around other stars can be detected if they are
large enough to cause the star to “wobble” as the planet
and star orbit around their common center of mass.
If the “wobble” is
longitudinal to
our line of sight,
it can also be
detected through
the Doppler shift
as the star's
motion changes.
Searching for Extrasolar Planets
We can observe
periodic Doppler
shifts of stars with
no visible
companion which
is evidence for the
wobbling motion
of the star around
the common
center of mass of a
planetary system
Searching for Extrasolar Planets
An extrasolar planet
may also be detected if
its orbit lies in the plane
of the line of sight to us.
The planet will then
eclipse the star, and if
the planet is large
enough, some decrease
in luminosity may be
observed.
Planets of Other Stars
Over 450 extrasolar planets have been found so
far and more are being found rapidly
• Most have masses comparable to Jupiter’s
mass
• Orbits are much smaller, and in some cases
very much smaller, than the orbit of Jupiter
• Orbits have high eccentricity
Evidence for
Ongoing
Planet
Formation
Many young
stars in the
Orion Nebula
are surrounded
by dust disks:
Probably sites
of planet
formation right
now!
Dust Disks
around
Forming
Stars
Dust disks
around some
T Tauri stars can
be imaged
directly (HST).
Planets of Other Stars
Orbits of 60 of the known extrasolar planets; note that
some of them are very close to their star.
Planets of Other Stars
Planets orbiting within 0.1 A.U. of their stars are called
“hot Jupiters”; they are not included in the previous
figure but are numerous.
Stars with composition like our Sun are much more
likely to have planets, showing that the “dusty disk”
theory of solar system formation is plausible.
Some of these “planets” may actually be brown dwarfs
(i.e. a large body not quite large enough to be a star),
but probably not many.
The other planetary systems discovered so far appear
to be very different from our own. Selection effect
biases sample toward massive planets orbiting close to
parent star; lower-mass planets cannot be detected this
way.
Planets of Other Stars
Recently, more Jupiter-like planets have been found; this
one has almost the mass of Jupiter and an orbital period
of 9.1 years.
The blue line is
the same curve for
Jupiter.
Planets of Other Stars
This figure shows
the size of the
habitable zone
(where there is a
possibility of liquid
water being present)
as a function of the
mass of the parent
star. Gliese 581c
and Gliese 581d are
close to this zone.